A widely employed method for manufacturing various inorganic sintered bodies includes the steps of mixing inorganic powder (e.g., alumina, silica, zirconia, mullite, silicon carbide, silicon nitride, and barium titanate) with a binder (e.g., various thermoplastic resins and organic compounds) to provide a green formed article, and firing the formed article to sinter the inorganic powder while decomposing and scattering the binder.
For example, for manufacturing a ceramic circuit board, a layered ceramic capacitor, or a separator for thin-layer chromatography, a ceramic green sheet formed in the shape of a thin film is used.
A ceramic green sheet is manufactured by uniformly mixing ceramic raw material powder, a binder resin, a plasticizer, a defoamer, a dispersant, an organic solvent, and the like using a mixer such as a ball mill to prepare slurry, applying the slurry to a support body, and removing the solvent by drying.
Especially, in the case of manufacturing a layered ceramic capacitor, the support body used is a PET film preliminary subjected to release treatment. After application of an electrically conductive paste that will serve as an internal electrode to a ceramic green sheet, the ceramic green sheet is peeled from the PET film as a support body. The resulting sheet is punched to give a piece in a predetermined size. A plurality of such pieces are stacked and heat-pressed to give a laminate, and then subjected to firing for removal of the binder resin by thermal decomposition. In this manner, a layered ceramic capacitor is manufactured.
Along with the recent trend toward miniaturization of electronic devices, a layered ceramic capacitor has been also desired to be miniaturized and to have a larger capacity. In a currently tried method for manufacturing a layered ceramic capacitor, ceramic powder having a smaller particle size (e.g., particle size of 500 nm or smaller) than conventional ceramic powder is used and 200 or more sheets of the resulting thin green sheets (e.g., 5 μm or less in thickness) are laminated.
A binder used in such a ceramic green sheet is desired to show higher performance than ever in the effect of increasing the strength of a ceramic green sheet to be manufactured and in thermal decomposability upon firing. Moreover, as described above, for lamination of 200 or more sheets of thin ceramic green sheets, favorable adhesiveness upon heat-pressing is important.
Patent Literature 1, for example, discloses a method of manufacturing a ceramic green sheet excellent in adhesiveness upon heat-pressing. The method uses polyvinyl acetal resins having different polymerization degrees in combination.
In a case where polyvinyl acetal is solely used as a binder resin, however, thermal decomposability is poor, though the sheet strength is good. Accordingly, the binder may not be completely decomposed and burned up and thus partly left as a residual carbide in the sintered body. Or, drastic decomposition and scattering of the binder in the sintering process may cause cracks, warping, swelling, or the like of a formed article.
The use of an acrylic resin, which has excellent thermal decomposability, is also considered. In this case, though the amount of a residual carbide after firing is reduced, a ceramic green sheet manufactured using an acrylic resin as a binder does not have sufficient strength or flexibility. As a result, in the process of drying the green sheet or in other subsequent processes, the green sheet unfortunately tends to have cracks.
To overcome the above problems, Patent Literature 2 discloses, as a binder that provides excellent sheet strength and flexibility, an acrylic binder having specifically defined resin characteristics (e.g., average molecular weight, acid value, glass transition temperature), for example. Patent Literature 3 and Patent Literature 4 individually disclose the use of an acrylic binder containing phthalate esters or the like as a plasticizer for the purpose of manufacturing a ceramic green sheet with flexibility.
Even in a case where the binder and the ceramic green sheet disclosed in the above literatures are used, when a thin ceramic green sheet having a thickness of 5 μm or less is to be manufactured, sufficient sheet strength or flexibility may not be achieved, problematically leading to damage of the ceramic green sheet upon peeling or punching.